
An optical coating is one or more
thin layers of material deposited on an optical component such as a
lens
A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...
,
prism or
mirror
A mirror, also known as a looking glass, is an object that Reflection (physics), reflects an image. Light that bounces off a mirror forms an image of whatever is in front of it, which is then focused through the lens of the eye or a camera ...
, which alters the way in which the optic
reflects and
transmits light. These coatings have become a key technology in the field of optics. One type of optical coating is an
anti-reflective coating
An antireflective, antiglare or anti-reflection (AR) coating is a type of optical coating applied to the surface of lens (optics), lenses, other optical elements, and photovoltaic cells to reduce reflection (physics), reflection. In typical ima ...
, which reduces unwanted reflections from surfaces, and is commonly used on
spectacle
In general, spectacle refers to an event that is memorable for the appearance it creates. Derived in Middle English from c. 1340 as "specially prepared or arranged display" it was borrowed from Old French ''spectacle'', itself a reflection of the ...
and
camera lens
A camera lens, photographic lens or photographic objective is an optical lens (optics), lens or assembly of lenses (compound lens) used in conjunction with a camera body and mechanism to Imaging, make images of objects either on photographic film ...
es. Another type is the high-reflector coating, which can be used to produce mirrors that reflect greater than 99.99% of the light that falls on them. More complex optical coatings exhibit high reflection over some range of
wavelength
In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats.
In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
s, and anti-reflection over another range, allowing the production of
dichroic
In optics, a dichroic material is either one which causes visible light to be split up into distinct beams of different wavelengths (colours) (not to be confused with dispersion), or one in which light rays having different polarizations are ab ...
thin-film filters.
Types of coating
The simplest optical coatings are thin layers of
metal
A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...
s, such as
aluminium
Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...
, which are deposited on glass substrates to make mirror surfaces, a process known as
silvering. The metal used determines the reflection characteristics of the mirror; aluminium is the cheapest and most common coating, and yields a reflectivity of around 88%-92% over the
visible spectrum
The visible spectrum is the spectral band, band of the electromagnetic spectrum that is visual perception, visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' (or simply light).
The optica ...
. More expensive is
silver
Silver is a chemical element; it has Symbol (chemistry), symbol Ag () and atomic number 47. A soft, whitish-gray, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. ...
, which has a reflectivity of 95%-99% even into the far
infrared
Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...
, but suffers from decreasing reflectivity (<90%) in the blue and
ultraviolet
Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...
spectral regions. Most expensive is
gold
Gold is a chemical element; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...
, which gives excellent (98%-99%) reflectivity throughout the
infrared
Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...
, but limited reflectivity at wavelengths shorter than 550
nm, resulting in the typical gold colour.
By controlling the thickness and density of metal coatings, it is possible to decrease the reflectivity and increase the transmission of the surface, resulting in a ''
half-silvered mirror''. These are sometimes used as "
one-way mirrors".
The other major type of optical coating is the
dielectric
In electromagnetism, a dielectric (or dielectric medium) is an Insulator (electricity), electrical insulator that can be Polarisability, polarised by an applied electric field. When a dielectric material is placed in an electric field, electric ...
coating (i.e. using materials with a different refractive index to the substrate). These are constructed from thin layers of materials such as
magnesium fluoride,
calcium fluoride, and various metal oxides, which are deposited onto the optical substrate. By careful choice of the exact composition, thickness, and number of these layers, it is possible to tailor the reflectivity and transmitivity of the coating to produce almost any desired characteristic. Reflection coefficients of surfaces can be reduced to less than 0.2%, producing an
''antireflection'' (AR) coating. Conversely, the reflectivity can be increased to greater than 99.99%, producing a ''high-reflector'' (HR) coating. The level of reflectivity can also be tuned to any particular value, for instance to produce a mirror that reflects 90% and transmits 10% of the light that falls on it, over some range of wavelengths. Such mirrors are often used as
beamsplitters, and as
output couplers in
laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...
s. Alternatively, the coating can be designed such that the mirror reflects light only in a narrow band of wavelengths, producing an
optical filter.
The versatility of dielectric coatings leads to their use in many scientific optical instruments (such as lasers,
optical microscope
The optical microscope, also referred to as a light microscope, is a type of microscope that commonly uses visible light and a system of lenses to generate magnified images of small objects. Optical microscopes are the oldest design of micros ...
s,
refracting telescope
A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens (optics), lens as its objective (optics), objective to form an image (also referred to a dioptrics, dioptric telescope). The refracting telescope d ...
s, and
interferometer
Interferometry is a technique which uses the '' interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber opt ...
s) as well as consumer devices such as
binoculars
Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. Most binoculars are sized to be held ...
, spectacles, and photographic lenses.
Dielectric layers are sometimes applied over top of metal films, either to provide a protective layer (as in
silicon dioxide
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundan ...
over aluminium), or to enhance the reflectivity of the metal film. Metal and dielectric combinations are also used to make advanced coatings that cannot be made any other way. One example is the so-called "
perfect mirror", which exhibits high (but not perfect) reflection, with unusually low sensitivity to wavelength, angle, and
polarization.
Antireflection coatings
Antireflection coatings are used to reduce reflection from surfaces. Whenever a
ray of light moves from one
medium to another (such as when light enters a sheet of
glass
Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
after travelling through
air
An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...
), some portion of the light is reflected from the surface (known as the ''interface'') between the two media.
A number of different effects are used to reduce reflection. The simplest is to use a thin layer of material at the interface, with an index of refraction between those of the two media. The reflection is minimized when
:
,
where
is the index of the thin layer, and
and
are the indices of the two media. The optimum refractive indices for multiple coating layers at angles of incidence other than 0° is given by Moreno et al. (2005).
Such coatings can reduce the reflection for ordinary glass from about 4% per surface to around 2%. These were the first type of antireflection coating known, having been discovered by
Lord Rayleigh in 1886. He found that old, slightly tarnished pieces of glass transmitted more light than new, clean pieces due to this effect.
Practical antireflection coatings rely on an intermediate layer not only for its direct reduction of reflection coefficient, but also use the
interference
Interference is the act of interfering, invading, or poaching. Interference may also refer to:
Communications
* Interference (communication), anything which alters, modifies, or disrupts a message
* Adjacent-channel interference, caused by extra ...
effect of a thin layer. If the layer's thickness is controlled precisely such that it is exactly one-quarter of the wavelength of the light in the layer (a ''quarter-wave coating''), the reflections from the front and back sides of the thin layer will destructively interfere and cancel each other.
In practice, the performance of a simple one-layer interference coating is limited by the fact that the reflections only exactly cancel for one wavelength of light at one angle, and by difficulties finding suitable materials. For ordinary glass (''n''≈1.5), the optimum coating index is ''n''≈1.23. Few useful substances have the required refractive index.
Magnesium fluoride (MgF
2) is often used, since it is hard-wearing and can be easily applied to substrates using
physical vapour deposition, even though its index is higher than desirable (n=1.38). With such coatings, reflection as low as 1% can be achieved on common glass, and better results can be obtained on higher index media.
Further reduction is possible by using multiple coating layers, designed such that reflections from the surfaces undergo maximum destructive interference. By using two or more layers, broadband antireflection coatings which cover the visible range (400-700 nm) with maximum reflectivities of less than 0.5% are commonly achievable. Reflection in narrower wavelength bands can be as low as 0.1%. Alternatively, a series of layers with small differences in refractive index can be used to create a broadband antireflective coating by means of a
refractive index gradient.
High-reflection coatings

image:Dielectric mirror diagram.svg, Diagram of a dielectric mirror. Thin layers with a high refractive index ''n''
1 are interleaved with thicker layers with a lower refractive index ''n''
2. The path lengths ''l''
A and ''l''
B differ by exactly one wavelength, which leads to constructive interference.
High-reflection (HR) coatings work the opposite way to antireflection coatings. The general idea is usually based on the periodic layer system composed from two materials, one with a high index, such as
zinc sulfide
Zinc sulfide (or zinc sulphide) is an inorganic compound with the chemical formula of ZnS. This is the main form of zinc found in nature, where it mainly occurs as the mineral sphalerite. Although this mineral is usually black because of various i ...
(''n''=2.32) or
titanium dioxide
Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound derived from titanium with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or Colour Index Internationa ...
(''n''=2.4), and one with a low index, such as
magnesium fluoride (''n''=1.38) or
silicon dioxide
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundan ...
(''n''=1.49). This periodic system significantly enhances the reflectivity of the surface in the certain wavelength range called
band-stop, whose width is determined by the ratio of the two used indices only (for quarter-wave systems), while the maximum reflectivity increases up to almost 100% with a number of layers in the ''stack''. The thicknesses of the layers are generally quarter-wave (then they yield to the broadest high reflection band in comparison to the non-quarter-wave systems composed from the same materials), this time designed such that reflected beams ''constructively'' interfere with one another to maximize reflection and minimize transmission. The best of these coatings built-up from deposited dielectric lossless materials on perfectly smooth surfaces can reach reflectivities greater than 99.999% (over a fairly narrow range of wavelengths). Common HR coatings can achieve 99.9% reflectivity over a broad wavelength range (tens of nanometers in the visible spectrum range).
As for AR coatings, HR coatings are affected by the incidence angle of the light. When used away from normal incidence, the reflective range shifts to shorter wavelengths, and becomes polarization dependent. This effect can be exploited to produce coatings that polarize a light beam.
By manipulating the exact thickness and composition of the layers in the reflective stack, the reflection characteristics can be tuned to a particular application, and may incorporate both high-reflective and anti-reflective wavelength regions. The coating can be designed as a long- or short-pass filter, a bandpass or notch filter, or a mirror with a specific reflectivity (useful in lasers). For example, the
dichroic prism
A dichroic prism is a prism (optics), prism that splits light into two beams of differing wavelength, wavelengths (colour). A trichroic prism assembly combines two dichroic prisms to split an image into 3 colours, typically as red, green and blue ...
assembly used in some
camera
A camera is an instrument used to capture and store images and videos, either digitally via an electronic image sensor, or chemically via a light-sensitive material such as photographic film. As a pivotal technology in the fields of photograp ...
s requires two dielectric coatings, one long-wavelength pass filter reflecting light below 500 nm (to separate the blue component of the light), and one short-pass filter to reflect red light, above 600 nm wavelength. The remaining transmitted light is the green component.
Extreme ultraviolet coatings
In the
EUV portion of the spectrum (wavelengths shorter than about 30 nm) nearly all materials absorb strongly, making it difficult to focus or otherwise manipulate light in this wavelength range. Telescopes such as
TRACE or
EIT that form images with EUV light use multilayer mirrors that are constructed of hundreds of alternating layers of a high-mass metal such as
molybdenum
Molybdenum is a chemical element; it has Symbol (chemistry), symbol Mo (from Neo-Latin ''molybdaenum'') and atomic number 42. The name derived from Ancient Greek ', meaning lead, since its ores were confused with lead ores. Molybdenum minerals hav ...
or
tungsten
Tungsten (also called wolfram) is a chemical element; it has symbol W and atomic number 74. It is a metal found naturally on Earth almost exclusively in compounds with other elements. It was identified as a distinct element in 1781 and first ...
, and a low-mass spacer such as
silicon
Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...
,
vacuum deposited onto a substrate such as
glass
Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
. Each layer pair is designed to have a thickness equal to half the wavelength of light to be reflected.
Constructive interference
In physics, interference is a phenomenon in which two coherence (physics), coherent waves are combined by adding their intensities or displacements with due consideration for their phase (waves), phase difference. The resultant wave may have ...
between scattered light from each layer causes the mirror to reflect EUV light of the desired wavelength as would a normal metal mirror in visible light. Using multilayer optics it is possible to reflect up to 70% of incident EUV light (at a particular wavelength chosen when the mirror is constructed).
Transparent conductive coatings
Transparent
conductive
In physics and electrical engineering, a conductor is an object or type of material that allows the flow of Electric charge, charge (electric current) in one or more directions. Materials made of metal are common electrical conductors. The flow ...
coatings are used in applications where it is important that the coating conduct electricity or dissipate
static charge
Static electricity is an imbalance of electric charges within or on the surface of a material. The charge remains until it can move away by an electric current or electrical discharge. The word "static" is used to differentiate it from curren ...
. Conductive coatings are used to protect the aperture from
electromagnetic interference, while dissipative coatings are used to prevent the build-up of
static electricity
Static electricity is an imbalance of electric charges within or on the surface of a material. The charge remains until it can move away by an electric current or electrical discharge. The word "static" is used to differentiate it from electric ...
. Transparent conductive coatings are also used extensively to provide electrodes in situations where light is required to pass, for example in
flat panel display technologies and in many
photoelectrochemical experiments. A common substance used in transparent conductive coatings is
indium tin oxide
Indium tin oxide (ITO) is a ternary composition of indium, tin and oxygen in varying proportions. Depending on the oxygen content, it can be described as either a ceramic or an alloy. Indium tin oxide is typically encountered as an oxygen-saturate ...
(ITO). ITO is not very optically transparent, however. The layers must be thin to provide substantial transparency, particularly at the blue end of the spectrum. Using ITO,
sheet resistances of 20 to 10,000
ohms per square can be achieved. An ITO coating may be combined with an antireflective coating to further improve
transmittance
Electromagnetic radiation can be affected in several ways by the medium in which it propagates. It can be Scattering, scattered, Absorption (electromagnetic radiation), absorbed, and Fresnel equations, reflected and refracted at discontinui ...
. Other TCOs (Transparent Conductive Oxides) include AZO (Aluminium doped Zinc Oxide), which offers much better UV transmission than ITO.
A special class of transparent conductive coatings applies to infrared films for theater-air military optics where IR transparent windows need to have (
Radar
Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...
) stealth (
Stealth technology
Stealth technology, also termed low observable technology (LO technology), is a sub-discipline of military tactics and passive and active electronic countermeasures. The term covers a range of military technology, methods used to make personnel ...
) properties. These are known as RAITs (Radar Attenuating / Infrared Transmitting) and include materials such as boron doped DLC (
Diamond-like carbon).
Phase correction coatings
The multiple internal reflections in
roof prisms cause a
polarization-dependent phase-lag of the transmitted light, in a manner similar to a
Fresnel rhomb
A Fresnel rhomb is an optical prism that introduces a 90° phase difference between two perpendicular components of polarization, by means of two total internal reflections. If the incident beam is linearly polarized at 45° to the plane of in ...
. This must be suppressed by multilayer ''phase-correction coatings'' applied to one of the roof surfaces to avoid unwanted
interference
Interference is the act of interfering, invading, or poaching. Interference may also refer to:
Communications
* Interference (communication), anything which alters, modifies, or disrupts a message
* Adjacent-channel interference, caused by extra ...
effects and a loss of contrast in the image. Dielectric phase-correction prism coatings are applied in a vacuum chamber with maybe 30 different superimposed vapor coating layers deposits, making it a complex production process.
In a roof prism without a phase-correcting coating, s-polarized and p-polarized light each acquire a different
geometric phase as they pass through the upper prism. When the two polarized components are recombined,
interference
Interference is the act of interfering, invading, or poaching. Interference may also refer to:
Communications
* Interference (communication), anything which alters, modifies, or disrupts a message
* Adjacent-channel interference, caused by extra ...
between the s-polarized and p-polarized light results in a different intensity distribution perpendicular to the roof edge as compared to that along the roof edge. This effect reduces contrast and resolution in the image perpendicular to the roof edge, producing an inferior image compared to that from a
porro prism erecting system. This roof edge diffraction effect may also be seen as a
diffraction spike perpendicular to the roof edge generated by bright points in the image. In technical optics, such a
phase is also known as the
Pancharatnam phase, and in quantum physics an equivalent phenomenon is known as the
Berry phase.
This effect can be seen in the elongation of the
Airy disk
In optics, the Airy disk (or Airy disc) and Airy pattern are descriptions of the best-focus (optics), focused Point source#Light, spot of light that a perfect lens (optics), lens with a circular aperture can make, limited by the diffraction of ...
in the direction perpendicular to the crest of the roof as this is a
diffraction
Diffraction is the deviation of waves from straight-line propagation without any change in their energy due to an obstacle or through an aperture. The diffracting object or aperture effectively becomes a secondary source of the Wave propagation ...
from the discontinuity at the roof crest.
The unwanted interference effects are suppressed by
vapour-depositing a special
dielectric coating known as a phase-compensating coating on the roof surfaces of the roof prism. These ''phase-correction coating'' or ''P-coating'' on the roof surfaces was developed in 1988 by Adolf Weyrauch at
Carl Zeiss
Carl Zeiss (; 11 September 1816 – 3 December 1888) was a German scientific instrument maker, optician and businessman. In 1846 he founded his workshop, which is still in business as Zeiss (company), Zeiss. Zeiss gathered a group of gifted p ...
[A. Weyrauch, B. Dörband: ''P-Coating: Improved imaging in binoculars through phase-corrected roof prisms.'' In: ''Deutsche Optikerzeitung.'' No. 4, 1988.](_blank)
/ref> Other manufacturers followed soon, and since then phase-correction coatings are used across the board in medium and high-quality roof prism binoculars
Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. Most binoculars are sized to be held ...
. This coating corrects for the difference in geometric phase between s- and p-polarized light so both have effectively the same phase shift, preventing image-degrading interference.
From a technical point of view, the phase-correction coating layer does not correct the actual phase shift, but rather the partial polarization of the light that results from total reflection. Such a correction can always only be made for a selected wavelength and for a specific angle of incidence; however, it is possible to approximately correct a roof prism for polychromatic light by superimposing several layers. In this way, since the 1990s, roof prism binoculars have also achieved resolution values that were previously only achievable with porro prisms.[Konrad Seil: Progress in binocular design. In: SPIE Proceedings. Band 1533, 1991, S. 48–60, doi:10.1117/12.48843] The presence of a phase-correction coating can be checked on unopened binoculars using two polarization filters.
Fano-resonant optical coatings
Fano-resonant optical coatings (FROCs) represent a new category of optical coatings.[ElKabbash, Mohamed, et al]
"Fano-resonant ultrathin film optical coatings"
Nature Nanotechnology, vol. 16, no. 4, pp. 440–446, 2021, Nature Publishing Group UK London. . FROCs exhibit the photonic Fano resonance by coupling a broadband nanocavity, which serves as the continuum, with a narrowband Fabry–Perot nanocavity, representing the discrete state. The interference
Interference is the act of interfering, invading, or poaching. Interference may also refer to:
Communications
* Interference (communication), anything which alters, modifies, or disrupts a message
* Adjacent-channel interference, caused by extra ...
between these two resonances manifests as an asymmetric Fano-resonance line-shape. FROCs are considered a separate category of optical coatings because they enjoy optical properties that cannot be reproduced using other optical coatings. Mainly, semi-transparent FROCs act as a beam splitting filter that reflects and transmits the same color, a property that cannot be achieved with transmission filters, dielectric mirror
A dielectric mirror, also known as a Bragg mirror, is a type of mirror composed of multiple thin film, thin layers of dielectric material, typically deposited on a substrate of glass or some other optical material. By careful choice of the type a ...
s, or semi-transparent metals.
FROCs enjoy remarkable structural coloring properties, as they can produce colors across a wide color gamut with both high brightness and high purity.[ElKabbash, Mohamed, et al]
"Fano resonant optical coatings platform for full gamut and high purity structural colors"
Nature Communications, vol. 14, no. 1, p. 3960, 2023, Nature Publishing Group UK London. . Moreover, the dependence of color on the angle of incident light can be controlled through the dielectric cavity material, making FROCs adaptable for applications requiring either angle-independent or angle-dependent coloring. This includes decorative purposes and anti-counterfeit measures.
FROCs were used as both monolithic spectrum splitters and selective solar absorbers, which makes them suitable for hybrid solar-thermal energy generation. They can be designed to reflect specific wavelength ranges, aligning with the energy band gap of photovoltaic cells, while absorbing the remaining solar spectrum. This enables higher photovoltaic efficiency at elevated optical concentrations by reducing the photovoltaic's cell temperature. The reduced temperature also increases the cell's lifetime. Additionally, their low infrared emissivity minimizes thermal losses, increasing the system's overall optothermal efficiency.
Sources
* Hecht, Eugene. Chapter 9, ''Optics'', 2nd ed. (1990), Addison Wesley. .
* I. Moreno, et al., "Thin-film spatial filters", ''Optics Letters'', 30, 914–916 (2005), .
* C. Clark, et al., "Two-color Mach 3 IR coating for TAMD systems", Proc. SPIE, vol. 4375, p. 307–314 (2001), .
References
See also
* List of telescope parts and construction
{{Authority control
Thin-film optics